In recent years, electronic devices have been downsized and packed with components at a higher density. This trend accompanies a strong request to use multi-layer boards not only in the industrial electronic field but also in the consumer electronic field. A denser populated multi-layer board, among others, depends on a multi-layering accuracy of multi-layered circuit patterns about its performance because the circuit-patterns become formed of finer pitches. Thus a higher multi-layering accuracy as well as a more productive multi-layering method is demanded.
A conventional method of manufacturing multi-layer boards, in particular, four-layer boards is described hereinafter. First, a method of manufacturing double-sided boards (two-layer boards) that is a base of multi-layer boards is described.
FIGS. 5A-5F show sectional views of the conventional manufacturing method of double-sided boards. FIG. 5A shows aramid-epoxy sheet 51 (hereinafter referred to as prepreg) and via 52. Prepreg 51 is formed of composite in which thermosetting epoxy resin is impregnated into non-woven fabric, of which dimensions are 400 mm square and 150 μm deep, formed of aromatic polyamide fiber. Via 51 is formed by filling a through hole, which is processed by laser machining, with conductive paste by printing. The prepreg is a composite formed of base material, such as glass-cloth or paper, impregnated with resin before undergoing a drying process to become a semi-hardened state.
FIG. 5B shows copper foil 53a, heater-punches 54a, 54b of which tip is approx. 10 mm across, mold-releasing sheet 55 formed of fluoro-resin, and positioning stage 56. Positioning vias (not shown) prepared to prepreg 51 are positioned on copper 53a by using a recognition device (not shown) such as CCD, then prepreg 51 is laminated thereon. Given places of prepreg 51 are pressed and heated by, via mold-releasing sheet 55, heater-punches 54a and 54b heated up to approx. 300° C. and with a pressure of 0.1 Mpa for approx. 3 seconds, thereby melting the thermosetting epoxy resin of prepreg 51 for rigidly bonding prepreg 51 to copper foil 53a. 
Next, as shown in FIG. 5C, the heat and press by heater-punches 54a, 54b are removed, then sheet 55 is peeled off. Reference mark 63 indicates recessed places pressed and heated by heater-punches 54a, 54b. 
Then as shown in FIG. 5D, laminate copper foil 53b on prepreg 51 such that prepreg 51 is sandwiched by foils 53a and 53b. Press and heat given places on prepreg 51 and copper foil 53a by heater-punches 54a, 54b heated up to approx. 100° C. with a pressure of 0.7 Mpa for approx. 3 seconds, thereby rigidly bonding foil 53b onto prepreg 51. As a result, upper and lower copper foils 53a, 53b are bonded to prepreg 51. The second press is done at the given places different from those of the first ones, so that solid bonding between respective copper foils 53a, 53b and prepreg 51 is completed.
Next, as shown in FIG. 5E, set a hot-press (not shown) at 200° C. and with 5 Mpa and heat and press prepreg 51 for approx. 2 hours with this hot-press, thereby melting and hardening the thermosetting epoxy resin for bonding upper and lower copper foils 53a, 53b solidly onto prepreg 51. At the same time, cut off surplus copper foil on end face 51T, thereby completing a laminated sheet having two layers of copper-clad. Upper and lower copper foils 53a, 53b are electrically coupled to each other by vias 52 filled with conductive paste.
Next, as shown in FIG. 5F, remove selectively copper foils from given places on the surface of copper-clad laminated board 57 by etching, and form circuit patterns 58, thereby completing board 59.
FIGS. 6A-6F show sectional views illustrating steps of a conventional manufacturing method of multi-layer boards, e.g. four-layer boards among others. In FIG. 6A, copper foil 53a and prepreg 51a are placed on positioning stage 56 undergone the same steps described in FIGS. 5A-5F.
Then laminate board 59 as an inner layer core-board onto prepreg 51a. This lamination is done after a positioning pattern (not shown) formed on board 59 is positioned with positioning vias (not shown) prepared on prepreg 51a by a recognition device (not shown) such as CCD.
After the lamination, press and heat prepreg 51 at given places via mold-releasing sheet 55 by heater-punches 54a, 54b heated up to approx. 300° C. for approx. 3 seconds. As a result, copper foil 53a, prepreg 51a, and board 59 are rigidly bonded to each other.
Next, as shown in FIG. 6B, detect and recognize positioning vias (not shown) of prepreg 51b by a recognition device such as CCD, then prepreg 51b is positioned with the positioning pattern formed on board 59 already fixed onto stage 56. After the positioning, laminate prepreg 51b onto board 59.
Next, press and heat board 59 at given places via mold-releasing sheet 55 by heater-punches 54a, 54b heated up to approx. 300° C. for approx. 3 seconds, thereby rigidly bonding board 59 and prepreg 51b to each other.
Then as shown in FIG. 6C, laminate copper foil 53b onto prepreg 51b, and press and heat foil 53b at given places by heater-punches 54a, 54b heated up to 300° C. for 3 seconds. This press and heat bonds lower copper foil 53a, lower prepreg 51a, board 59, upper prepreg 51b, and upper copper foil 53b respectively to each other, and they are fixed to each other.
Next, as shown in FIG. 6D, set the hot-press (not shown) at 200° C. with 5 Mpa, and press and heat the entire laminated unit including board 59 for approx. 2 hours by the hot-press. This press and heat melts the thermosetting epoxy resin included in upper and lower prepregs 51a, 51b, thereby rigidly bonding upper and lower copper foils 53a, 53b, upper and lower prepreg 51a, 51b to each other with the thermosetting epoxy resin. As a result, four-layer and copper-clad board 60 is produced.
Further, as shown in FIG. 6E, remove selectively the copper foil from given places on the surface of four-layer copper-clad board 60 by etching, and form circuit patterns 61, thereby completing multi-layer board 62 having four layers.
If more layers are needed to multi-layer board 62, repeat the steps discussed above using four-layer board 62 shown in FIG. 6E as a core board.
Prior art including what is discussed above is disclosed, e.g. in Japanese Patent Application Non-Examined Publication No. H07-283534.
FIG. 7 shows a schematic diagram illustrating problems in the prior art. Mold-releasing sheet 55 is disposed between heater-punch 54 and prepreg 51. This placement allows pressing and heating prepreg 51 at given places free from dirtying or damaging heater-punch 54. This placement also allows the resin included in prepreg 51 to be completely hardened for bonding prepreg 51 to copper foils 53a, 53b. However, the given places pressed and heated are still kept at a high temperature, and thermosetting epoxy resin 62 included in prepreg 51 is still in a melting state. Since prepreg 51 is still welded with sheet 55, parts of resin 62 melted is attached to or absorbed by sheet 55 when sheet 55 is peeled off.
Sections 63 recessed due to the press and heat at a high temperature have less resin because the resin is melted to flow out, so that the core member of prepreg 51 is exposed. In other words, sections 63 are recessed due to the press and heat by heater-punch 54, and have little resin because the resin is melted at a high temperature and pressed, whereby the resin is pushed away.
The resin pushed away is attached to mold-releasing sheet 55, and removed, so that recessed sections 63 become short of resin. The resin still remained there has been hardened almost completely, so that uniform thickness in resin cannot be expected even if some resin flows in from the vicinity in an onward step using a hot-press. Therefore, recessed sections 63 are left in a porous state, so that etching solution tends to enter into recessed sections 63.
In other words, according to the conventional manufacturing method, prepreg 51 is pressed and heated at a high temperature when prepreg 51 is laminated and rigidly bonded onto the copper foil or a core board, so that the resin at the pressed section flows out and is completely hardened. As a result, the core member of prepreg 51 is exposed and left in a porous state. The users have been obliged to accept this inconvenience.
If undesirable porous sections occur on prepreg 51, etching solution enters into those sections when circuit patterns are formed, and residue of the etching solution adversely influences the remaining manufacturing steps. The users also have been obliged to accept this inconvenience. In other words, the conventional manufacturing method has a problem that the residue of etching solution causes pollution to the steps onward.
In the conventional lamination step, mold releasing sheet 55 and prepreg 51 solidly attached to sheet 55 pull each other, so that the materials laminated slide from each other.
The present invention provides a method of manufacturing substrates, which method hardens the prepreg resin incompletely at the lamination and is excellent in accuracy as well as productivity. The present invention also provides a mold-releasing sheet and an apparatus for manufacturing boards, and a method of manufacturing boards using the same mold-releasing sheet and the same manufacturing apparatus.